Telephone system



June 29, 1954 SELDEN 2,682,576

TELEPHONE SYSTEM Filed Jan. 20, 1951 5 Sheets-Sheet 2 INVENTOR.

June 29, 1954 SELDEN 2,682,576

TELEPHONE SYSTEM Filed Jan. 20, 1951 5 Sheets-Sheet 3 BUSY TONE INVENTOR.

June 29, 1954 l. H. SELDEN ,68

TELEPHONE SYSTEM Filed Jan. 20, 1951 5 Sheets-Sheet 4 BUSY 759 TONE 758 g 75'! U/AL 70/145 06 v 7 6'05) TONE REF INVENTOR.

June 29, 1954 Filed Jan. 20, 1951 l. H. SELDEN TELEPHONE SYSTEM 5 Sheets-Sheet 5 INVENTOR.

Patented June 29, 1954 UNITED STATE; RATENT OFFICE 10 Claims.

This invention relates to telephone systems and particularly to small automatic intercommunh eating systems, and refers particularly to that type of service supplying central ofiice telephone terminations of one or more trunks or local switchboard lines at several different stations or extensions in an oflice or group of onices and also supplies automatic telephone intercommunication in which the selection of the code numbers of the various stations is done by the keying or push button method. Heretofore systems. have been proposed in which the telephone designations assigned to telephone lines or trunk circuits have been selected by means of transmitting direct cur-rent impulses, usually by employing a dial. Inter-office selection of telephone code numbers has been speeded up by employing keying senders combined with register relays and timing means which together release impulses to operate the central office switches at a distant exchange. I

This system contemplates the application of the trunk pick up keys at the stations operated in combinations to select code numbers after a local or intercom button has been operated which will energize a transfer relay that will transfer the trunk pairs at a common equipment cabinet, to select leads in the intercommunication system. Since the stations in such an arrangement usually are close to each other the same cabling that is provided to connect the various trunks to the stations is utilized by means of the transfer relay. Thus by employing the large amount of conductors available a simplified selecting a1- rangement is made possible. However, the central oiiice connections are not indispensable to the operation of the intercommunicating circuits nor vice versa.

The primary objects of this invention are to provide a cheaper, simpler, more reliable and faster means of intercommunication in a twelve and thirty-six station system. In push button selection there is a novel modern advancement in communication appealing to a modernistic public eager for new and faster means of communication.

l-Ieretofore, telephone systems of the push button selecting type have usually used buttons and conductors not associated with the trunk conductors and the stations were wired directly to each other, whereby selection was made by one button only and the talking circuit consisted of a single talking link frequently common to all stations.

Other more modern automatic telephone syss ill tems use a dial which is slow and tedious to operate and is usually associated with very complicated selecting circuits which are not always reliable or cheap to maintain. An audible signal is usually used which is operated by an interrupted ringing current which causes further delay after the called number has been dialed.

In this system the stations are assigned code numbers which are selected by operating combinations of 2 or 3 of the trunk pick up buttons. In the twelve station system 2 digit numbers are used and in the 36 station system 3 digit numbers are used and immediately after selecting the last digit, the audible signal at the called station starts, thus elminating every possible delay inv completing a call.

Other objects of the intercommunicating systcm are: to provide a local talking circuit such that a calling party While talking may be signalled on his local circuit and upon completion of his first conversation can depress his local button and accept the next call; and

To provide an intercommunicating circuit in accessible to other stations without special pickup features. In designing an automatic thirty-six station system, a field is touched that is insufficiently covered at the present time. This system also provides a local circuit adaptable to many optional features or modifications and imposes no handicap on the trunk circuit arrangements, thus giving the utmost of flexibility to the stations connected or grouped in the intercommunieating arrangement.

This system also proposes an automatic trunk lock-out feature whereby only one station can have access to an outside trunk at one time. This feature is optional but is imperative to any telephone system that has'several central ofiice dial trunk terminations and extensions, for it not only insures privacy but prevents calling wrong numbers due to interruptions while one subscriber is dialing and another subscriber lifts the receiver of a station which may be connected to the same trunk the subscriber is dialing on.

The invention has been illustrated in connection with the following drawings in which:

Fig. 8 shows diagrammatically the numerical grouping of the locals according to their first digit for selection and by their last digit for talking.

Figs. 1 to 7 show the various features of the invention applied in detail to a small capacity automatic telephone system.

Fig. 1 shows the wiring of the pick up key circuit, the local calling buzzer and the extra switchhook contact, which is part of an instrument. There is no instrument shown.

Fig. 2 shows the trunk lock-out relay.

Fig. 3 shows the trunk busy relay.

Fig. 4 shows a battery supply lead to be used in the trunk lock-out circuit of any stations that may not be connected to the intercommunicating equipment.

Fig. 5 shows the station relay circuit of one station, together with its trunk transfer relay and talk link transfer relay.

Fig. 6 shows inpart the selecting circuit of the twelve station system common to all, together with a talk link busy lamp, a talk link busy relay and the selecting link busy relay.

Fig. 7 shows in part the selecting relays for a thirty-six station system together with a talk link busy lamp, a talk link busy relay and the selecting link busy relay.

Fig. 8 shows schematically how the locals are grouped numerically by their first digits for push button selection and by their last digit numerically for the talk links. Fig, 8 is abbreviated to facilitate a simple description of the selecting process to give a ready overall understanding of the mechanism.

Fig. 9 is a schematic to simplify description and show how the trunk circuit would connect if no intercommunicating circuit were required.

Fig. 10 is a block diagram showing how the figures are related to each other.

The busy tone circuit and dial tone circuit are not shown. Any tone circuit is adaptable that has low resistance to ground on the dial tone circuit and high resistance to ground on the busy tone circuit. It must be such a circuit as will start or furnish dial tone when battery is imposed on the dial tone lead or busy tone when battery is imposed on the busy tone lead.

In a 12 station system the local code numbers are 12, 13, 14, 21, 23, 24, 31, 32, 34, 41, 42, 43. There can be no numbers such as 11, 22, 33, or 44 because in most present types of key equipment the keys are so constructed that a pick up key will stay depressed until a subsequent key is pressed, thus no double code numbers can be used.

In Fig. 8 the leads 101, 102, 103, 104, and 105 terminate at a telephone represented by 801 consisting of a telephone circuit connected to 4 central office or switchboard lines and one local line by means of 5 pick up keys which may be either internally or externally mounted with the telestation circuit.

The leads IOI, I02, I03, I04, and I05 represent cable pairs and with relays 530 and 553i both normal the local station can have access to any of 4 central ofiice trunks or switchboard lines 802.

With the receiver off the hook when the fifth or intercommunicating button is depressed ground is fed out on lead I05 and operates relay 53L Relay 53I operated transfers leads IIJI, I02, I03, and I04 from central ofiice circuits to select leads 68L 602, 803, and 604 and prepares the operating path for relay 530.

Relay 53I operated locks operated under control of relay 530 and an extra switchhook contact in the telephone set. This circuit is not shown.

As is apparent in Fig. 8, local 12, 8I2, l3, 8I3, and 14, 8M, are grouped under control of relay 65I which might be designated VI (signifying vertical group 1 or 10s group). Relay 652 would be designated V2 for vertical group 2 or s, 653 would be V3 and 054 would be V4.

Thus if the calling party now wished t 6 station 12, M2, he would then depress trunk key 1. By so doing battery is placed on lead I0 I which would then operate relay BSI, VI. Relay 65I operated locks under control of relay 53I, circuit not shown, and prepares a select path to station relay circuits of stations, 12, 13, and 14 represented by BIZ, 8 I3, and 8I4 after passing through link busy test circuits represented by 803.

The calling party next depresses the trunk 2 button associated with lead I02 which operates a relay in the station circuit of station 12, 8I2. This causes another relay in said station circuit to operate completing operating circuit of relay 530 and operates an audible signal at called station. Relay 530 (talk link transfer relay) operated releases relay 53I and G5I and transfers leads IOI, I02, I03, and I04 from the selecting link to talk links EM, 502, 563 and 504 respectively.

With the operation of a line relay in the sta tion circuit of BIZ when the receiver is lifted at said station, said circuit is connected to talk link 2, 502, and since the calling party has just pressed trunk button 2 his station circuit is also on talk link 502. Independent battery feeds supply talking battery and the talking circuit is established with 2 condensers in the called station circuit.

To make possible this talk link arrangement the stations are grouped according to their last digit for the four talk links thus, SM, 824, and 834 are on talk link 4, 504. Also 8I3, 823 and 843 are on link 3, 503. BIZ, 832 and 842 are on link 502 and MI, 831, and B ll are on talk link L Thus it can be seen there are 4 horizontal talk links with the stations grouped according to their last digits and 4 vertical select groups in which the stations are grouped according to their first digits.

The system as disclosed in Figs. 1 to 8 comprises an automatic push button selecting arrangement which utilizes the extensions of central office trunks or local switchboard lines from their common equipment by transferring these said leads from trunk circuits to selecting leads and talking lead which terminate in a common group of relays and by the use of code numbers it is possible to select any one of 12 stations in the first case or 36 stations in the second case by the use of dif ferent number combinations of the 4 pick up keys at any station in the system. Thus it can be seen the trunk conductors together with the pick up push buttons are utilized in a simplified selecting circuit, in which only one station can place an inter-office call at one time. However, this process is rapid, taking only long enough to press 2 or 3 buttons as the case may be, then the selecting circuit is released for the next call. The 12 station system requires only 2 digit code numbers while the 36 station system requires 3 digit code numbers.

The flexibility of this equipment is brought out by the fact that in a 12 station system it would be possible for as high as 48 different outside trunks to terminate in the equipment without any effect whatever on the intercommunicating features. Any station may have access to from none to four outside lines. Any station may either be equipped with a dial in dial exchanges or not equipped whichever the needs of the service would require.

To pick up another stations local calls, all that is required is a transfer key to pick up the local circuit, that .is, the tip and ring of any stations local line.

connects with lead 662 at relay 65!. through lower outer back contacts of relay 656,

coded 1, 2, 3, i, and local and are shown as I2I,

I22, I23, I24 and I25.

To illustrate the operation of the circuit in making a station to station call it will be assumed that the subscriber at any station such as shown in Fig. 1, desires to communicate with station 3I2, the code number of which is 12. To do this he operates local key I and removes the receiver, not shown, from the switchhook, not shown, of the station circuit II2. Trunk transfer relay 53!, which is of the slow to release type, operates on a circuit traced from battery on the winding of relay 53!, through an upper back contact of 33!), through the lower outer back contact of 5.32, over lead I83 through inner lower front contact of key I 25, through normally made back contacts of remaining pick up keys and enters the station circuit I12 on lead I23, through the station circuit and out on lead I2? through upper inner back contacts of first 4 pick up key and upper front contact of key I25, over lead I85, upper outer back contact of relay .53 I, lower middle back contact of relay 532, lead 368 to back contact of relay 655, to ground on lead 633 through a repeating coil, not shown, in the common tone circuit 651. Relay 53! operated locks under control of an extra switchhook contact II5 of the calling station circuit.

The circuit for this function is traced from ground through a resistance 523. This resistance is sufiicient so as not to shunt the sound of dial tone from lead I nor release dial tone relays. Through lower outer front contact of relay 53!, outer lower back contact of relay 533, over lead I I4, outer lower front contact of pick up key I25, through a chain or series circuit of the other pick up keys to extra switchhook contacts H5 to the ring side of the local pair lead I33 thence over the original operating path of relay 53! to battery, holding relay 53I operated under control of the switchhook or relay 533.

With the operation of relay 53! lead I05 at upper outer swing contactof relay 53'! is transferred from lead 638 to lead 63%. Lead 606 is also a dial tone lead, consequently the calling station will receive dial tone since his station circuit is still in series with the operating path of relay 53! previously described.

With the operation of relay 53! leads IOI, I02, 133 and I84 are connected through to leads 60!, 382, E03, and 33 3 respectively. Leads I38, I39, III} and Ill are now connected with the battery standing on the winding of relay 53 I.

Wishing to select code number 1, the calling station now depresses pick up key I2! which op crates relay in the following manner.

Ground on the winding of relay 35! over lead through .a lower front .contact 53! of relay 53! to lead IflI. Key I2! is operated. Into the station circuit over lead I2! out on 523 which is connected with lead I38 which has battery standing on it from contact of relay 53!.

Relay locks through its inner contact to lead 635, which has battery on it from upper outer back contact of relay .533. Relay 65! prepares the selecting path for stations l2, l3 and 14 shown as BIZ, Bit and 3M.

The next digit selected is 2 by depressing key I22. In pressing key I22 battery from a contact of relay 53! to lead I39 through the station circuit is fed out over lead I02 to lead 632. Lead 362 over lead 5I2, to ground through winding of relay 534.

Consider Figs. 1 and .5 to be that of both the calling station and the called station. In a completed call relay 53.0 is the only relay operated in the calling station circuit. In the called station circuit the line and cut off relays 533 and 532 are the only relays operated, thus in considering Figs. 1 and 5 as being equipment for both the called party and the calling party the only common part would be Fig. 1.

The calling stations station circuit will be on pick-up key I22 and the called station will be on pick-up key 125.

Relay 53 operates, closes the operating path of relay .532. Relay 533 also operates relay .535! of the calling station. Lead 535 multiples at all relays 533 and all relays 533. The relay 533 of the calling station is the only one the mate of which the transfer relay 53! is operated since only one subscriber can originate a call at one time.

Relay .53! prepared the operating path of relay 533 by placing ground on one side of the winding of relay .530 at an upper contact 532 of relay 53!, consequently the relay 533 of the calling station is the only one that will operate. Relay 533 operates and releases relay 53! in opening back contact 543 of relay 533. Relay 53!) transfers the locking ground 523 from relay 53! to its lower outer front contact which locks relay 53!} under control of the extra switchhook contact the same as previously described for relay 53 I.

Relay 530 releases relay 35!, upper outer contact of relay 533 and lead 335. Relay 55! released, releases relay 533. Relay 533 transfers the selecting leads to the four talk links 58!, 532, 553, and 534. Relay 533 operated also releases the select link circuit so that the next station can place a call. The calling station l I2 with associated select key I22 operated is now bridged across leads I32 and I39, which are now connected at relay 5'33 with talk link 532. Consequently the calling station I I2 is bridged across talk link 532. Relay 532 of the called station circuit operated, locks operated by leads 533 and 50'! and the station IIZ bridged across said talk link.

Operation of cut off relay 532 of the called station opens the operating path of relay 53! at the two lower swing contacts of relay 532 and prepares the operating circuit to the line relay 533 in the called stations circuit in cutting leads I35 and IE8 through from intercommunicating pick-up key I25 of the called station.

The operation of relay 532 operates a buzzer, such as H3 at the called station, by a circuit traced from ground through buzzer II3 over lead I01, upper outer back contacts of relay upper middle front contacts of relay 532 to battery.

This schematic drawing implies that direct current relay battery is used. However, 60 cycle alternating current which is preferable could be used. If the needs of the service require, a bell could replace the buzzer and 20 cycle generator current could replace the battery at upper middle front contacts of relay 532.

The party at the called station answers by depressing the local or intercommunicating button represented by I25, Fig. 1. In so doing his station such as I I2 is bridged across the leads N5 and N36 associated with station code 12.

This station bridge operates relay 533. This circuit is traced from battery on the upper winding of relay 533 over lead H1 to lead I06 through lower outer front contact of relay 532. Through the station bridge from lead 155 back on Hi5 through upper outer back contact of relay 53! through a lower middle front contact of relay 532 to lead 555 to ground on the lower winding of relay 533. Relay 533 operated opens the operating path of the buzzer circuit to the called station at lead H11 upper outer back contact of relay 533. Relay 533 operated locks operated under control of relay 532. 77

This locking circuit is traced from battery on the upper winding of relay 533, through upper inner front contact of said relay, through resistance 5H through upper inner front contact of relay 532 to lower winding of relay 533 to ground.

The object of this locking bridge is to prevent a recall at the called station in case the called station should restore his receiver first. The resistance of resistance SM is sufilcient to hold relay 533 operated when the receiver is restored at the called station and sunicient to not shunt down the talking circuit when the called subscriber is talking.

When both subscribers are connected and talking the calling station is supplied with talking battery from relay 532 of the called stations relay group. The called station gets talking battery from its associated relay 533. The talking circuit is completed with condensers 512 and 513.

Station to station call, select link busy If a subscriber attempts to make a call when the select link is busy a previously operated relay 53! would still be operated. A. relay 53! operated, closes the operating circuit of relay 555, in the following manner. Battery on the Winding of relay 555, over lead 597 to ground on an upper contact of relay 554. The operation of relay 655 transfers lead 558 from dial tone lead 636 to busy tone lead 655. Thus lead 688 will now have busy tone on it as will all leads multipled with lead 888 at relays 532. As before mentioned each station connected with the common equipment and wired for station selection will also be equipped with each of the 5 station relays shown in Fig. 5.

Thus each of the 12 relays 532 would have busy tone on its contact 5M. Consequently if a subscriber at any other station at this moment were trying to select another station he would receive only busy tone instead of dial tone and could not operate his relay 53! since the resistance to ground through busy tone is greater than the amount of resistance to ground through dial tone.

Station to station call, talk. link busy Since there are only four talk links it can be observed that in a twelve station system there would be three stations on each talk link as far as incoming calls are concerned. However if one station in a talk link group is connected on an incoming call the other two stations may still originate calls but cannot receive any incoming calls as long as their called talk link is busy. Although each group of three has a common talk link the conversations on said talk links are absolutely private for there is no way another subscriber may have access to said talk link when it is busy unless he has a transfer key especially wired to pick up calls of another station.

To prevent a double connection each talk link 8 has a talk link busy relay such as relay 656. Relay 556 as shown is the talk link busy relay for talk link 2, 582. As before stated the stations on talk link 582 are 812. 832 and 852 code numbers 12, 32 and 42.

In Fig. 6, it can be seen that a ground on any one of three leads, 609, 5]] or 522 would operate relay 856. Lead 6! l is associated with the sta tion circuit of 8-32, not shown. Lead 522 is associated with the station circuit of 852, not shown, and lead 659 is associated with station circuit 842 which may be considered as the station circuit shown in Fig. 5.

If the station circuit of 8E2 as here represented were busy as a result of having received a call then relay 532 would be operated as previously described. Upper outer contact of relay 552 places a ground on lead 609 and operates relay 556. It is obvious that any subsequent calls to talk link two would come over vertical select relays 625i, 653 or 554 and eventually over leads 662, 612 and 682. With relay E operated, then leads B62, 672 and 682 would all have a busy tone standing on them since lead 560 is a busy tone lead connected to the common tone circuit 657.

Since all selections are made with battery on the ring side of the selecting circuit in series with said station circuit out over the tip of the line, then with the selection of the second digit no matter which of the three code numbers may have been selected the tip of the station circuit would be connected to ground through busy tone and the calling station would receive a busy tone signal.

When a station originates a call the 53s relay is the only station relay in its own group that remains operated and since the local button is released with the subsequent pressing of the next select key the subscriber is not on his local circuit when his call is completed but on the circuit or talk link of the last button he has pressed. Consequently he may still receive an incoming call on his local circuit. Relays 532 and 533 remain operated in the called station circuit during conversation and are held under control of the calling stations bridge.

Busy lamps S58 and 2'58 as shown in Figs. 6 and 7 are self explanatory and may be connected to any talk link busy relay to show which station circuits are not clear to accept calls.

Although Fig. 8, in simplifying the selecting circuit diagrammatically shows the grouping of the stations by their first digit in vertical groups, Fig. 6, is apt to be confusing in that after passing relays 65L 552, 653 and 654 the select leads are shown grouped by their last digit to simplify the schematic circuit extending to the talk link busy relays. 893 represents the grouping of all relays 656 in Fig. 8, however Fig. 8 is a true picture of the numerical grouping of the stations. In Fig. 6, boxes 655l, 6553 and 655-4 represent talk link busy relays similar to relay 656 with their respective select leads 162i E3! and etc.

The thirty-six: station system up keys numbering l to 4, it is necessary to use ent combinations from four keys so numbered without any double numbers being permitted due to the construction of most pick up keys.

For selection the code numbers are grouped by their first digits thus: 121, 123, 124, 131, 132, 134, 141, 142, and 143.- The second group: 212, 213, 214, 231, 232, 234, 241, 242, and 243. The third group: 312,. 313,. 314, 321., 323, 324, 341, 342 and 343. The fourth group: 412, 413, 414, 421, 423, 424, 431, 432 and 434. Refer to Fig. 7 and compare to Fig. 6. Since the selecting principle is the same, then it follows that all numbers the first digit of which is 1 will be selected through relay 75! which is similar to relay 65 I. li'he second group each number of which has 2 for its first digit will be selected through relay 52. The threes group through relay T53 and the fours group through relay I54.

Since we now have three digit numbers as compared to two digit numbers in the twelve station system it follows there will have to be an additional selecting step which numerically has to be an intermediate one. It follows then that in each vertical group as determined by the first digit, see Fig. 8, there will have to be three succeeding divisions made in which. the numbers are grouped according to their second digit.

As previously stated, numbers 412, 413, 414 and 421, 423, 424, and 431, 432, and 434, all will be selected through relay 154. By grouping these numbers by their second digits as just listed we have our basis for selection of the intermediate digits. Consider leads WI, 102, 103, I04 and 105 to be the same as leads Bill, 602,. 633, 304 and 535 and it follows that relay iSI may be operated under control of select or pickup key I21, relay i52by key I22, relay 153 by key I23 and relay 1'54 by key I24.

Referring now to the fourth vertical group suppose that a number is being selected and key i24 has been depressed and relay it is locked operated, it is now obvious with relay hi4 operated, that relays 14' I, I42 and 743 can be operated under control of pick up buttons IN, I 22, and I23 and leads WI, 102 and T03 respecti'velyl It follows that code numbers 412', 413 and 414 will be selected through relay 74 I. Numhers 421, 423 and 424 through relay T42 and numbers 431, 432 and 434 through relay 743.

When the second digit relay has been selected" and. looked through its winding to lead 535, the third digit is selected, the same as the second digit in the twelve station system. For after the numbers are grouped by the second digit for the intermediate selecting step they are then regrouped by their last digit for the four talk links, thus: IZl, HI, I4l, 23I, 24!, 32I, MI, 421, 43k are connected to talk link I and these leads must pass through a talk link busy relay such as shown in Fig. 7, relay 556, before passing over a station select lead such as H2 which is the same as 3E2. Lead 112 would select and operate the station relays of station 432. Relay E56 is the talk link 2' busy relay.

Stations I32, I42, ZIZ, 232', 242, 3I2, 342, 412' and 432 are grouped on talk link 2 and their cor responding select leads pass through the contacts of the talk link 2 busy relay shown as 756. station select leads I23, I43, 213, 243,. 3I3, 323', 343 M3 and 423, are in group 3 and I24, I34, 21s, 23s, sis, 324, 41-4, 424 and 434 are in group 4.

In Fig. 7,. l? and 159 represent the common tone circuit and: lead 169 is a busy tone lead at I59. Relay IBI is a ground release relay placed inthe 3 digit system to prevent subsequent operation of a second and third vertical relay or subsequent relays upon selection of the last two digits.

The function of relay 'l6I is described as follows: If the first digit selected were a l, by operation of key #IZI, then. relay 35! would operate. its operating path would be from battery on lead 134, through the winding of relay WI through a left hand middle back contact of relay "i5I over lead T3 to ground through contacts of relay iBI.

Relay lEi operated operates relay 'iSI in a circuit traced from ground on left hand front contacts to battery through the winding of relay L Relay 'lEiI operated transfers from its operating path to a locking path which releases lead 17! to be subsequently used in the selecting of the last digit of three code numbers in this vertical group. The locking path is traced from battery on lead which is the same as lead 663,v through right hand inner front contact of relay 15!, through the winding of relay T5! to ground through left hand outer front contacts of said relay. Since battery on lead 635 is under control of relay 530 then relay i5! is looked under control of relay 530 and relay 16! is locked operated under control of relay I5 I. Relay it! operated disconnects the ground from lead Tit. Consequently there can be no subsequent opera- "tion of any of. the vertical group relays after the first one has operated since the operating and locking paths of all said relays are the same. This was deemed. necessary in the first step of the 36 station system on account of the three digits but does not seem'practical" for the 12 station system or the second digit of the 36 station system.

In Fig. 7, boxes I73, I74 and H5 represent the select leads and second digit relays such as the drawing discloses above first digit relay I54.

At relay I33 leads 103' and H? are similar to leads 333 and BIZ in Fig. 6. Lead H2 is the select Ieadfor. code number 432. Lead I32 is the select lead for code numberfl32 and lead I42 for code number 142 etc. Their mate leads 132' and I42" etc.,.have the same function aslead 109.

LeadsiAfI and 43I are select leads for codenumbers 421 and 431 but must pass through a link busy relay similar to relay 133, but for talk link'I. Leads M3 and 423 pass through a similar relay as do leads 414, 424 and 434.

In the 36 station system there are nine stations per talk link. However this only effects incoming calls since outgoing calls have no connection with the incoming talk link.

Central office trunk: calls; 12 and 36 station systems With relays 530 and 53! normal, central ofiice trunk calls may come and go the same as if there were no automatic intercommunicating equip ment, with the exception of the central ofiice trunk lock out feature, which is optional with this equipment.

Central olfice trunk lock-out feature 563 etc., then through the trunk lock out relay Fig. 2. Thus, the abovementioned leads 504, I, 583 etc, would connect to leads 204, 2H, 263 etc., respectively. For simplicity of the circuit description however, Fig. 2 has been drawn adjacent to Fig. 1, since the circuit operation will be the same as if Fig. 5 were not considered.

If a subscriber at a station such as 112, wished to place an outside trunk call and operated key Hi and lifted the receiver from the switchhook he would operate relay 231 in a circuit traced as follows: ground from the back contact of relay 3 35, over lead 3m through back contact 225 of relay 23 5, over lead 201 and lead mi through the bridge of the station circuit H2 back on lead iflii to 2538 through contact 222 of relay 231, through the winding of relay 23l thence over lead 2H5, lead lid to lower middle back contacts of pickup key i25 through a series circuit of all the pick up keys to the extra switchhook contact Ht of the station circuit to lead Hit through lower outer contact of relay 532 through contact 548 of relay 536 through the winding of relay 53i to battery. In this circuit the ratio of the resistances of the relays 53f and 23l is such that in this series circuit relay 23l will operate while 53l will not. Relay 231 operates and locks to ground on its upper inner front contact back over lead 2M, H t the series circuit through the pick up keys to lead 106 and battery on the winding of relay 531. In the operation of relay 23, contact 223 must make before any other back contact of said relay breaks, as any two of the back contacts could be in the operating circuit of said relay.

Relay 23% connects all the trunk leads through to central oiiice, however the only trunk pair that has a station bridge on it is on leads iti and H33 now connected to 302 and 383. Leads am, 382, and 303 of Fig. 2, pass on to leads EM, 362 and 303 of Fig. 3. Leads 3M, 302', 303' etc., of Fig. 2 will also connect to a relay such as shown in Fig. 3. Leads 391 and 368 designate a line to a central office or to a local switchboard. With the station bridge across leads 392 and 3133 relay 385 operates from ground at the central ohice on lead 3t2 through the winding of relay 3535, through the station bridge back on lead 353 to battery at central office. Relay 305 operated places busy tone on lead 361 at front contact of relay 305 to lead 310 which is a busy tone lead oi the common tone circuit 306. Each group of three wires such as 39!, 302 and 383 from relay 2 23 is wired to a separate trunk busy relay 3%.

Central ofiice call, trunk: busy Each station equipped with the trunk lock out feature would require one Fig. 2. Any station not connected to the automatic intercommunicating equipment would require a Fig. 4, or a battery supply through a resistance to take the place of the battery furnished at the 536 relay on lead m6 of stations that are connected to the automatic equipment. Each trunk would require one Fig. 3.

If a succeeding station were to choose a trunk that was already in service, then the relay 365 of that trunk would already be operated and busy tone would be on the common lead 38L Consequently the calling party would receive busy tone in a circuit which is the same as the circuit that operated relay 23! except that the ground at relay 365 is now a ground through a high resistance with a busy tone superimposed thereon.

Consequently the calling party would receive busy tone at his station circuit since it would be in series with the operating path of relay 231.

12 Relay 23! can not operate however due to the laugh resistance to ground in the busy tone circuit Series make and break circuit at pick up keys It can be seen that at the pick up keys such as l2! the springs could be adjusted so that when a key is depressed its spring it! in swinging from the back contact to the front contact would for an instant cause an open in the locking circuits of relays 231 and 530. In the case of relay 535 it has been specified as being of the slow to release type. Consequently this instantaneous break would not affect the locking circuit of relay 53l if said relay were operated as is the case when a local code number is being selected, however in the case of relays 23! and see this instantaneous break in the locking circuit has a definite purpose in the circuit which is to release said relays which must be adjusted to release rapidly.

The purpose of this function is as follows: upon completion of an outside trunk call or an intercom call in which the relays 23i or 533 have been operated a subscriber may desire to place another call. Sometimes it is more expedient to press another trunk button than to operate the switchhook in a normal manner, to disconnect the last call and originate a new one.

If the make and break arrangement were not incorporated in the locking circuit of these relays then said relays would not release while transferring the station circuit from one pick up key to another, consequently it would thus be possible to seize another circuit which might be busy. Therefore it is necessary to have the make and break arrangement in the pick up key series circuit of the locking circuit of said relays to insure the release of same between each key selection.

I claim:

1. In a telephone system, subscribers stations, each station equipped with a switch hook and an extra switch hook contact, four trunk pick up keys at said stations, an intercommunicating key at said stations, a trunk transfer relay per station, trunks and station pick up key leads and selecting link leads terminating in said trunk transfer relays, said transfer relays under control of station circuits at said subscribers stations and a signal lead in a series circuit through contacts at all said trunk pick up keys and said extra switch hook contact, a selecting link circuit terminating in four select leads at said trunk transfer relays and consisting of four first digit relays and twelve second digit or supervisory relays, the four select leads associated respectively with the four trunk pick up keys, two digit code numbers, a selecting link busy relay, a link trans- .i'er relay per station, select link leads and talk link leads and station pick up key leads terminating in said link transfer relay, four talking links, a talk link busy relay for each talking link, three station group relays per station consisting of, a line, cut-oi? and a supervisory or last digit relay, a talking circuit consisting of two condensers and battery and ground feeds; means under joint control of said signal lead, said switch hook, said extra switch hook contact and the intercommunicating key to transfer the trunk leads, extending from said trunk transfer relays to the trunk pick up keys at said stations, from trunk circuits to select leads in the said selecting link circuit, means further responsive to the operation of a trunk pick up key to operate a first digit relay and means responsive to the operation of a second trunk pick up key to operate a second digit or called stations supervisory relay; means further responsive upon operation of a called stations supervisory relay to operate the calling stations link transfer relay to transfer said calling station from the selecting leads to the talking links to operate the line relay of the called stations station circuit to operate an audible signal at the called station; means further responsive upon removal of the receiver at the called station when the intercommunicating button has been operated to operate the cut oii relay of the called station to stop the audible signal at said called station and to establish a talking cir cult between the calling and the called stations.

2. In a telephone system, subscribers stations, each station equipped with a switch hook and an extra switch hook contact, four trunk pick up keys at said stations, an intercommunicating key at said stations, a trunk transfer relay per station, trunks and station pick up key leads and selecting link leads terminating in said trunk transfer relays, said trunk transfer relays under control of station circuits at said subscribers stations and a signal lead in a series circuit through contacts at all said trunk pick up keys and said extra switch hook contact, a selecting link circuit terminating in four select leads at said trunk transfer relays and consisting of four first digit relays and twelve second digit relays and thirty-six last digit or supervisory relays, the four select leads associated res ectively with the four trunk pick up keys, three digit code numbers, a selecting link busy relay, a link transfer relay per station, select link leads and talk link leads and station pick up key leads terminating in said link transfer relay, four talking links, a talk link busy relay for each talking link, three station group relays per station consisting of, a line, cut-on and a supervisory or last digit relay, a talking circuit consisting of two condensers and battery and ground feeds; means under joint control of said signal lead, said switch hook, said extra switch hook contact and the intercommunicating key to transfer the trunk leads, extending from said transfer relays to the trunk pick up keys at said stations from trunk circuits to select leads in the said selecting link circuit, means further responsive to the operation of a trunk pick up key to operate a first digit relay and means responsive to the operation of a. second trunk pick up key to operate a second digit relay; means further responsive to the operation of a third trunk key to operate a third digit or called stations supervisory relay; means further responsive upon operation of a called stations supervisory relay to operate the calling stations link transfer relay to transfer said calling station from the selecting leads to the talking links and to operate the line relay of the called stations station circuit to operate an audible signal at the called station; means further responsive upon removal of the receiver at the called station when the intercoinmunicating button has been operated to operate the out off relay of the called station to stop the audible signal at said called station and to establish a talking circuit between the calling and the called stations.

3. In a telephone system, subscribers stations, four trunk pick up keys at said stations, an intercommunicating key at said stations, a trunk transfer relay per station, trunks and station pick up key leads and selecting link leads terminating in said trunk transfer relays, a selecting link circuit terminating in four select leads at said trunk transfer relays and consisting of four first digit relays and twelve second digit relays or/and thirty-six last digit or supervisory relays the four select leads associated respectively with the four trunk pick up keys, two or three digit code numbers, a selecting link busy relay, four talking links, a talk link busy relay for each talking link, a link transfer relay per station, select link leads and talk link leads and station pick up key leads terminating in said link transfer relay, three station group relays per station consisting of, a line, cut-off and a supervisory or last digit relay, a talking circuit consisting of two condensers and battery and ground feeds, four talking links one-fourth of the stations grouped in each talk link group according to their last digits in a numbering arrangement whereby one quarter of the stations are in a group in which the last digit of each number is 1, another quarter of the stations are so grouped as to have 2 for their last digit, another group has all code numbers ending in 3 and another group has all numbers ending in l; means responsive when selecting a code number, to the operation of the last trunk pick up key in the selection of the called stations last digit when the calling stations link transfer relay has been operated to connect the calling station through the trunk pick up key circuit on a talk link common to all stations in that group.

4. In a telephone system, subscribers stations, two or three digit code numbers, trunk pick up keys at said stations, an intercommunicating key at said stations, a trunk transfer relay per station, trunks and station pick up key leads and selecting link leads terminating in said trunk transfer relays, a selecting link circuit terminating in four select leads at said trunk transfer relays and consisting of four first digit relays and twelve second digit relays or thirty-six last digit or supervisory relays, a link transfer relay per station, select link leads and talk link leads and station pick up key leads terminating in said link transfer relay, a selecting link busy relay, dial tone leads and busy tone leads terminating in said relay and a tone control lead terminating in said relay and extending to the operating paths or" all said trunk transfer relays, said busy tone lead having greater resistance to ground than the dial tone lead; means responsive to the operation of an intercommunicating key with the receiver off the switch hook to receive dial tone and operate the associated trunk transfer relay to extend the trunk pick up key leads to the selecting link circuit when said link circuit is not busy and means further responsive to the operation of a trunk transfer relay to operate said selecting link busy relay whereby another subscriber upon operation of a subsequent intercommunicating key with the receiver oif the switch hook will r ceive busy tone and the associated trunk transfer relay will not operate.

5. In a telephone system, subscribers stations, trunk pick up keys at said stations, an intercoinmunicatingkey at said stations, a trunk transfer relay per station, trunks and station pick up key leads and selecting link leads terminating in said trunk transfer relays, a selecting link circuit terminating in four select leads at said trunk transfer relays and consisting of our first digit relays and twelve second digit relays or thirty-six last digit or supervisory relays, two or three digit code numbers, a selecting link busy relay, a link transfer relay per station, select link leads and talk link leads and station pick up key leads termlnating in said link transfer relay, three station group relays per station consisting of, a line, cut-off and a supervisory or last digit relay, four talking links, a talk link busy relay for each talking link, busy tone leads and selecting leads terminating in said talk link busy relays the control lead or each said talk link busy relay terminating in the station out 01f relays associated with each talk link group and a talk link busy lamp per each talking link associated with said talk link busy relay; means responsive to the operation of a called stations cut off relay to operate said talk link busy relay to place busy tone on the select lead of that talk link group whereby a subsequent station in selecting a, station in said busy talking link will receive busy tone when the last digit has been selected; means to prevent access to said busy talk link; means to indicate visually that the associated talk link is busy.

6. In a telephone system, subscribers stations equipped with a switch hook and an extra switch hook contact, trunk pick up keys at said stations, an intercormnunicating key at said stations, a

trunk transfer relay per station, trunks and station pick up key leads and selecting link leads terminatin in said trunk transfer relays, a selecting link circuit terminating in four select leads at said trunk transfer relays and consisting of four first digit relays and twelve second digit relays or thirty-six last digit or supervisory relays, two or three digit code numbers, a link transfer relay per station, select link leads and talk link leads and station pick up key leads terminating in said link transfer relay, a trunk busy relay per trunk, busy tone leads terminating in said trunk busy relay, a trunk lock out relay per station, trunks and trunk pick up key leads and busy tone leads terminating in said trunk lock out relays, a signal lead in a series circuit through contacts at all said trunk pick up keys and said extra switch ho contact; mean under joint control of said signal lead said switch hook and said extra switch hook contact to operate said trunk lock out relay to extend the trunk pick up key leads through to the trunks; means further responsive to operate said trunk busy relay to extend said busy tone to a subsequent subscriber choosing the same trunk, and to prevent access to same trunk.

'7. In a telephone system, subscribers stations, said stations equipped with a switch hook and an extra switch hook contact, trunk pick up keys at said stations, a trunk busy relay per trunk operative on a series winding with said trunk, busy tone leads terminating in said trunk busy relay, a trunk lock out relay per station, trunks terminating in said lock out relays, trunk pick up key leads and busy tone leads terminating in said trunk lock out relays, a signal lead in a series circuit through contacts at all said trunk pick up keys and said extra switch hook contact; means under joint control of said signal lead said switch hook and said extra switch hook contact to operate said trunk lock out relay to extend the trunk pick up key leads through to the trunks; means further responsive to operate said trunk busy relay to extend said busy tone to a subsequent subscriber choosing the same trunk, and to prevent access to same trunk.

8. In a telephone system, subscribers stations, said stations equipped with a switch hook and an extra switch hook contact, trunk pick up keys at said stations, a trunk busy relay per trunk, busy tone leads terminating in said relay, a trunk look out relay per station, trunks and trunk pick up key leads and busy tone leads terminating in said trunk lock out relays, a signal lead through said extra switch hook contact and a series circuit through make and break contacts at all trunk pick up keys, said make and break contacts adjusted to break before making; means under joint control of said signal lead said switch hook and said extra switch hook contact to release an associated trunk lock out relay without restoring the receiver to the switch hook when changing from one trunk selection to another by operating a subsequent trunk pick up key.

9. In a telephone system, subscribers stations, each station equipped with a switch hook and an extra switch hook contact, four trunk pick up keys at said stations, an intercommunicating key at said stations, a trunk transfer relay per station, trunks and station pick up key leads and selecting link leads terminating in said trunk transfer relays, said transfer relays under control of station circuits at said subscribers stations and a signal lead in a series circuit through contacts at all said trunk pick up keys and said extra switch hook contact, a selecting link circuit terminating in four select leads at said trunk transfer relays; means under joint control of said signal lead, said switch hook, said extra switch hook contact and the intercommunicating key to transfer the trunk leads, extending from said trunk transfer relays to the trunk pick up keys at said stations, from trunk circuits to select leads in the said selecting link circuit.

10. In a telephone system, subscribers stations, each station equipped with a switch hook and an extra switch hook contact, four trunk pick up keys at said stations, an intercornmunicating key at said stations, a trunk transfer relay per station, trunks and station pick up key leads and selecting link leads terminating in said trunk transfer relays, said transfer relays under control of station circuits at said subscribers stations and a signal lead in a series circuit through contacts at all said trunk pick up keys and said 1 extra switch hock contact, a selecting link circuit terminating in four select leads at said trunk transfer relays and consisting of four first digit relays and twelve second digit or supervisory relays, said four first digit relays under control or the said four select leads associated respectively with the four trunk pick up keys, said twelve second digit relays under control of said pick up keys and said first digit relays, two digit code numbers, a selecting link busy relay, four talking links a link transfer relay per station, select link leads and talk link leads and station pick up key leads terminating in said link transfer relay, a talk link busy relay for each talking link, three station group relays per station consisting of, a line relay, a cut-oi? relay and a supervisory or last digit relay, said link transfer relay under control of a called stations supervisory or last digit relay and the calling stations trunk transfer relay; said cut oil relay under control of s id supervisory relay a talking circuit consisting of two condensers and battery and ground feeds associated with a called stations station circuit, said line relay under control of said called stations station circuit and an intereommunicating key at said station, an audible signalling device at said called station under control of said cut-off relay and said line relay, said called stations talking circuit always associated only with the respective talking link of said called station; means responsive to the 17 operation of a trunk pick up key, subsequent to the operation of a trunk transfer relay, to operate a first digit relay and means responsive to the operation of a second trunk pick up key to select the talk link associated with the called station and to operate a second digit or called stations supervisory relay; means further responsive upon operation ofa called stations supervisory relay to operate the calling stations link transfer relay to transfer said calling station from the selecting leads to the talking links and to operate the out 01f relay of the called stations station circuit to operate an audible signal at the called station; means further responsive upon removal of the receiver at the called station when the intercommunicating button has been operated to operate the line relay of the 18 called station to stop the audible signal at said called station and to establish a talking circuit between the calling and the called stations. 

